Frequency shift apparatus



July 29, 1952 c. J. MADSEN FREQUENCY SHIFT APPARATUS Filed NOV. 10, 1948 INVENTOR Carl J.

2E mmoa 95m SzE 528mm of; one embodiment of the invention.

Patented July 29, 1952 UNITED sm-res mm;for s;1

. 7, I t r 2,605,359

- FRE UENCY SHIFT APPARATUS ;J. Madsen, .Hingham, Mass., assignor to I Westinghouse Electric Corporation, East Pittsf burgh, Pat, a corporation ofBennsylvania Application November 10, 1948', Serial No. 59,266

This invention relates to radio teletypewriter systems utilizing frequencyshift keying.

The conventional teletypewriter system utilizing frequency-shift keying requires polarized relays for operating the selector magnets of the teletypewriter printer, and requires; involved push-pull amplifier circuits for operatingthe relays. 1 t I I This invention provides a radio teletypewriter system utilizing frequency-shift keying in which the selector magnet isoperated directly from the anode circuit of an amplifier tube,v and in which but two tubes are used between the discriminator and the selector magnet. In one embodimentof the invention, the discriminator acts to apply ,a

negative bias on the grid of a control tubewhen the Si nalis on the mark frequency, causing an amplifier tube to become conductive and to energize or close the selector magnet of the printer. .When the signal is on the space frequency, the

discriminator acts, to apply a positive bias to the grid of the control tube, causing the amplifier tube to become non-conductiva so that the selector magnet of the printer is deenergized or released. During the period of no signal, the grid of the amplifier tube is so biased that it draws sufficient anodecurrent to barely maintain ,the selector 'magnet in closed position at the threshold of operation. i

While in normal operation, a mark signal is transmitted during periods .Of'llO trafiio; however, occasional component casualties may'occur I in either the transmitter or receiver units which will result in a no signal condition.

An object of the invention is to provide for the direct operation of. the selector magnet in. the printer of a radio teletypewriter system utilizing frequency-shift, keying, from the receiver-converter unit without the use of relays.

Another object of the invention is to reducethe number of vacuum tubes and other components in; radio teletypewriter systems utilizing frequency-shift keying. 1

Another object of the invention is to provide a radio teletypewriter system utilizing frequencyshift keying in which the selector magnet current is maintained at a valuebarely sufficient to hold the armature of the selector magnet in closed position during the periods no signalsare being received e The invention will now be described withreference to the drawing which is a circuit schematic The conventional receiver I supplies received l ,and'limiter I2, into The outputs of the two discriminator rectifier tubes l4 and iii are connected in polarity oppositionacross the series-connected resistors 16 and ii, which have such values that at cross-over frequency the potentials across the two resistors are equal and opposite. The junction point of the resistors l6 and i"! is connected totheanodes ofthe tubes l and I5 through lead 30, resistor 3| and resistors 32 and 33. It is seen that the end-of resistor 33 opposite the junction point with resistor 32 is connected to the anode of the tube i i, and the end of resistor 32 opposite the junction point with resistor 33 is connected to the anode of tube ii. The resistor 3| functions to maintain a substantially constant load on the limiterlZ.

The resistors l6 and I7 are'connected across the input of the low-phase filter l8, the output of which is connected to the grid of thecontrol tube 9 and to the negative terminal of the anode voltage supply source 20. p

The anode of thetube I9 is direct-connected to the control grid of the amplifier tube 2|, and the cathode of the tube I9 is connected to the slider of the potentiometer 22, one end of which is connected to the cathode of the tube 2|, and the other end of which is connected to the negative side of the source 20.

The anode of the tube I9 is connected through the resistor 23 to the positive side of the source '23, and its grid is connected through the grid resistor 28 to' the negative side of the source 20.

The screen grid of the tube 2| is connected through the resistor 24 to the positive side of the source 2|). The cathode of the tube 2| is con'-' 'nected through the resistor 25 to the positive side plied to the grid of the contro1 tube l9. The potentiometer 22 is adjusted for varying the-bias on the grid of the tube 2| until the anode current of the tube 2| is approximately to of normal value. Thus, lack of signal will maintain sufi'icient current through the selector magnet winding 25' to prevent the teletypewriter printer from runningopen as would ordinarily occur when no signal is being received.

When the incoming signal is on the mark frequency, the rectifier tube l5 will conduct, causing the grid of the tube I9 to be biased negatively with respect to its cathode so that it is nonconducting, with no current flow through the resistor 23, thus permitting the control grid of the tubeffl to become'positive with res'pect to its cathode." The tube 2| is then fully conductive and draws normal anode current which passes through the selector magnet winding 26, provid-- -trol-g'rid or the'tube'Zl to become negative with respect to its cathode, resulting in the tube being non-conductive, and resulting in a'decrease in the anode current of thetube 2| which causes the Winding '25- of the selector magnet 'to be deenergized.

1 Thus, whenno signai is being received, the selector magnet currentis maintained ata value barely su fficie'nt to hold the armature of'the'selector magnet inclosed position at the threshold "of operation. When the signal is on the mark frequency, the selector magnet armature is closed; and when the signal is on the space frequency," the selector magnet is deenergized so that it is placed in open position. I claim as my invention:

l. A radio teletypewriter receiver for receiving signals transmitted on space and mark frequenci's', comprising atherr'nionic control tube having 'a grid, a cathode, and an anode; a thermionic amplifier tube having a control grid, a'cathode,

and an anode; an anode voltage supplys'ource; a printer selector electromagne't having an energizii'i 'g Winding connected in Series with said "source and said anode of said amplifier tube; a

fre uency discriminator; circuit-connections connesting the output of'said discriminator to said grid and cathode of said control'tuoa'saidconnections including means for bias'ing said grid of said control tube to one polarity with'respect to its cathode when" signals on mark-frequency are supplied into'said discriminator, and for'biasing said grid of said control tube to the opposite polarity with respect'tb its cathode when signals on space fre'quencyare supplied into said discriminatoi; circuit connections connecting said g 'fidiand cathode of said amplifier tube to said anode and cathodeof said control tube, said lastmentioned connections including means for biasing said grid of said amplifier tube positively with respect to its'c'a'thode when said gridof said control tube is biased negatively, whereby suffieient anode current drawn by said amplifier tube will flow through said magnet winding for placing said magnet in its closed position, and for biasing said grid of said amplifier tube negatively with respect to its cathodewhen said-gridof said control tube is biased positively, whereby the i is anode current of said amplifier tube will be re- 1 duced so as to deenergize said winding, whereby said selector magnet will be placed in its open position.

2. A radio teletypewriter receiver-as claimedin claim 1 in which the 'last-mentioned-circuit connections includes means for normally biasing "tive with resp'ect 'to its cathode that sufiicient anode current will flow through said winding for maintaining said magnet in closed position at the threshold of operation when no signals are applied to said discriminator, and in which a iow pa'ss' filter is located between the output of the discriminator' 'and' the-grid and cathode of the control tube.

54-A radio 'telcty'pewr'i'ter receiver for receiving Signals transmitted' oh' space and mark frequen- "cies; eomp'rising a fre uency discriminator'inc'ludirig two recti'fiertubes having cathodes and anodes; a control tube having a grid, a cathode and ananode; an amplifier tube having'a-grid, a;"cath6d'ej and an anode; a cathode bias resistor connected to the cathode of said eontre -tube'and totl ije-cathode of'said' arriplifier tube; circuit connections'c'onnecting said grid of said control tube to the cathode of one of said rectifier tubes, and "connecting the cathode of the other of'sai'd rectifierftub'es to said bias resistor; resistorscon- 'neet'e'd in series between Said cathodes- 0f said rectifier. tubes, the mid-point c'orlnectionbf Said 'ser i's -connctedresistors being connected to said anodes of said rectifier tubes, said series c'onn'ected resistors'bein'g so connected and having suchvalues that at the cross-over frequency, the potentialsthereacross areequal in valueand are 'oppositein polarity, said anode of said control tube b'ein'g direct connected'to" said grid of said amplifier tube; an'anode voltage supply source; a grid bias resistor conriectin'g'said grid of'said control tube to the negative terminal of -said source, and a printer selector e1ectromagnet having its energizing winding connected inseries with said supply sourc'e and said anode of said amplifier'tube. I I

s. "A radio t'el'etypewriter receiver"as claimed in claim 5 in which'alow pass'filterislocated betweenthe series-connected resistors and the grid and. cathode of said control tube.

7. A' radio teletypewriter "receiver as "claimed in claim 5 in which theicathodebias resistoris a potentiometer having one end connected 'to the negative terminal of *s'aidisource, having its other -'end'-' connected to 's'aid cathode "of said amplifier tube, and having its sliderconnected to said cathode of said control tube.

ha per Name Date 2,112,595 29 1933 2,230,681 Frischlinecht -Feb.4, 1941 2,406,034 Phelps Aug, 20,1946 2,507,730 "Mitchell 'M'ay 16-, "1950 

